A container medium is a mix of a diversity of materials that can be used for growing plants. A container medium (also named substrate, potting mix, plant growing media, potting soil, etc.) generally comprises one or more light weight neutral aggregates (silica sand, perlite, vermiculite, expanded polystyrene, etc.), and an organic constituent (Sphagnum peat, composts, etc.), optionally with soil. The amounts included of the aforementioned ingredients vary widely among various container media, depending on the container size, the irrigation system and their ultimate utilization.
Composts prepared from heterogeneous wastes and used in container media may be naturally suppressive against a variety of diseases caused by soil-borne plant pathogens, or may become so upon the addition of antagonistic microorganisms. This suppressive effect can be highly variable. Compost and compost-amended media are usually more suppressive than Sphagnum peat-media because of the populations and activities of the microbial communities that are able to sustain. Composts can provide energy source for a great diversity of microorganisms and facilitate the biological control of plant pathogens. However, when Sphagnum peat is used as a sole organic component in a plant growth media, this media is a poor energy source and usually do not suppress plant diseases.
Two frequent diseases of plants are wilts and damping-off caused by some fungi (Fusarium oxysporum, Verticillium spp., Rhizoctonia solani). Fusarium wilts are diseases caused by several f. sp. (formae specialis, special forms) and races of Fusarium oxysporum. These wilts cause severe losses on many crops, specially in high temperature regions, and they are difficult to be controlled by chemicals (often the highly contaminant methyl bromide has to be used). Rhizoctonia damping-off is one of the most serious diseases of nursery crops throughout the world. Some specific antagonistic microorganisms, especially members of the genera Pseudomonas and Trichoderma, have been used to suppress the above-mentioned diseases. However, in practice it has proved to be very difficult to ensure that these antagonistic microorganisms in the plant growing media do achieve sufficient population densities to effectively suppress the plant disease. Consequently, in order to render a suppressive plant growth media for agricultural purposes, there is a need for new methods to produce or ensure sufficient populations of desired microorganisms. In this sense, artificial inoculation of container media formulated preferably with compost and antagonistic microorganisms is more suitable. Finally, in view of the significant losses caused to various commercial crops by Fusarium oxysporum and Rhizoctonia solani, it is highly desirable a method of reproducibly producing container media which is suppressive to both pathogens.
Some attempts to biologically control plant diseases are known where Trichoderma spp. are used. Thus, for instance, U.S. Pat. No. 4,900,348 discloses a method for producing a container medium which is suppressive to diseases caused by Rhizcotonia solani and Pythium ultimum which employs a mixture of Trichoderma hamatum isolate 382 and Flavobacterium balustinum isolate 299. U.S. Pat. No. 4,642,131 discloses a method where Trichoderma hamatum is used in combination with bacteria. However, none of these methods allows the control of Fusarium wilts what is an important limitation, given that in practice these wilts have to be mainly controlled by using methyl bromide, a highly hazardous chemical which causes serious environmental problems.